Pseudomonas aeruginosa is an environmental Gram-negative bacterium, associated with a broad spectrum of infections in humans. It has a very large genome which is remarkably flexible metabolically, explaining why it can be found in very diverse environments. This opportunistic pathogen can cause acute lung injury and mortality through the delivery of exotoxins by the type III secretion system (TTSS).
The Type III Secretion System (TTSS) of P. aeruginosa is a complex multi-protein structure crossing the complete cell wall. It is a specialised hollow needle-like molecular structure secreting only TTSS proteins and pathogenicity related toxins. Many different proteins form the TTSS, both on the bacterial cytoplasmic side and externally. Externally, only the single ‘barrel’ homopolymeric forming protein and the ‘needle tip’ protein are accessible to antibodies. The needle protein PcrV is thought to form a ring-type structure on the tip of the needle. The TTSS complex can inject various exotoxins, produced by the bacterium, directly into the cytoplasm of host cells.
The involvement of this translocation apparatus in pathogenesis may not be limited to the transport of exotoxins, as indeed mutants expressing TTSS but not the toxins are cytotoxic as well (Lee et al. Infect. Immun. 73: 1695-1705, 2005). The translocation pore itself is sufficient to cause the death of host cells, either directly through pore-mediated increases in membrane permeability, or indirectly through the activation of broad cellular defense responses. The TTSS virulence mechanism on the bacteria's external surface enables P. aeruginosa to evade human immune defenses by killing white blood cells and epithelial cells and triggering tissue-damaging inflammation.
Under normal circumstances, the bacterium is perfectly harmless. However, under certain circumstances, the bacterium can colonize hosts with a weakened immune system. It is recognized as a major cause of nosocomial bacteremia and infections associated with invasive devices, mechanical ventilation, burn wounds, or surgery in the immunocompromised and the immunocompetent patients (Giamarellou and Kanellakopoulou Crit. Care Clin. 24: 261-278, 2008), such as bone marrow transplant patients (Velasco et al. Clin. Microbial. Infect. 10: 542-549, 2004). P. aeruginosa typically causes nosocomial infections of the pulmonary tract, urinary tract, (burn) wounds and also sepsis.
In Cystic Fibrosis (CF) patients, P. aeruginosa infection follows a well-established pattern of recurrent pulmonary infection in early childhood leading to the establishment of chronic infection in older CF patients, where it is a major contributing factor in the progressive decline in lung function and disease exacerbations leading to respiratory failure (FitzSimmons J. Pediatr. 122: 1-9, 1993; Kerem et al. J. Pediatr. 116: 714-719, 1990; Lyczak et al. Clin. Microb. Rev. 15; 194-222, 2002). Once chronic P. aeruginosa pulmonary infection is established, eradication of the organism appears impossible using current therapies (Lee Chronic Respiratory Disease 6: 99-107, 2009).
P. aeruginosa has several different manifestations in the setting of chronic obstructive pulmonary disease (COPD). The organism is a colonizer that is cleared quickly, causes acute exacerbations and also may cause chronic infections in a subset of adults with COPD (Murphy Curr. Opin. Pulm. Med, 15: 138-142, 2009).
A good overview on the current treatment of P. aeruginosa pneumonia is given by Giamarellou and Kanellakopoulou (Crit. Care Clin. 24: 261-278, 2008), Malcolm and Heim (Curr. Opin. Pharmacol. 9: 558-565, 2009), El Solh and Alhajhusain (J. Antimicrobial Chemotherapy 64: 229-238, 2009) and Roux and Ricard (Infectious Disorders—Drug Targets 11: 389-394, 2011). Current treatment for patients still relies on antibiotics. Antibiotics of the four major structural classes are in use against P. aeruginosa infection (Giamarellou and Kanellakopoulou Crit. Care Clin. 24: 261-278, 2008). Importantly, once P. aeruginosa colonization has been established, it cannot be successfully cleared using antibiotics due to biofilm formation. Biofilm limits the access of certain antibiotics to the deeper layers of the film (diffusion limiting). More importantly, the deeper layers of biofilm contain many P. aeruginosa bacteria which are live but virtually completely inactive for lack of nutrient access. Antibiotics of various classes act on cell division or highly active metabolic pathways, and are thus unable to kill these dormant bacteria. Once therapy is tapered back or withdrawn, these cells rapidly re-colonize the patient.
Furthermore, P. aeruginosa has the ability to evade new antimicrobial therapies and develop resistance, being on one hand intrinsically resistant to many drugs, on the other hand rapidly acquiring resistance via a number of mechanisms (Malcolm and Heim Curr. Opin. Pharmacol. 9: 558-565, 2009). Because of the versatility and the large size of P. aeruginosa genome, various resistance mechanisms can be present simultaneously, causing cross-resistance to several antipseudomonal agents (Giamarellou and Kanellakopoulau Crit. Care Clin. 24: 261-278, 2008). Novel variants on the same basic antibiotic structures are in development and may alleviate current resistance to some extent, but are very likely to give rise to novel resistance once in widespread clinical use. No novel classes of antibiotics are known to be in clinical development. Because the development of new classes of antibiotics has lagged far behind our growing need for such drugs, we now face a post-antibiotic era with limited capacity to combat these infections.
Topical administration of existing antibiotics (e.g., aerosol administration of tobramycin or colistin) has been used to deliver higher local concentrations of antibiotics without exposing the patient to high systemic levels which may be toxic to the patient (Luyt et al. Curr. Opin. Infect. Dis. 22: 154-158, 2009 However, continued concerns are raised about its efficacy and potential emergence of resistance as well (El Solh and Alhajhusain J. Antimicrobial Chemotherapy 64: 229-238, 2009; Roux and Ricard Infectious Disorders—Drug Targets 11: 389-394, 2011).
With the pipeline of new antimicrobial agents running dry, treatment of P. aeruginosa continues to rely on the theoretical advantages of combination therapy and the revival of old drugs previously abandoned because of serious toxicity, like polymyxins (Giamarellou and Kanellakopoulou Crit. Care Clin. 24: 261-278, 2008; El Solh and Alhajhusain J. Antimicrobial Chemotherapy 64: 229-238, 2009). However, resistance to such treatment is rapidly emerging with very worrisome latest resistance rates, and the appearance of Pseudomonas strains with multidrug-resistant, or even pan-resistant, phenotypes (Malcolm and Heim Curr. Opin. Pharmacol. 9: 558-565, 2009). Based on the reported resistance surveillance data, it is evident that the current therapeutic approach for P. aeruginosa infections is approaching its limits (Giamarellou and Kanellakopoulou Crit. Care Clin. 24: 261-278, 2008).
There are currently no non-antibiotic based treatments on the market. However, there are a number of drug candidates in development.
Various monoclonal antibodies, mostly directed to P. aeruginosa flagellin or strain-specific LPS have been described. Most did not reach clinical stage. One LPS-reactive IgM (Kenta (Berna/Crucell) is listed as in active Phase II development. However, the serotype specificity of this antibody underscores the need for a quick assay to determine the serotype of the infectious agent in the hospital setting and the development of antibodies specific for other clinic-relevant serotypes (Roux and Ricard Infectious Disorders—Drug Targets 11: 389-394, 2011).
A mouse monoclonal anti-PcrV antibody, monoclonal antibody (Mab) 166, with potent neutralizing activity in mouse and rat models of Pseudomonas infection had been described by Frank et al. (J. Infect. Dis. 186: 64-73, 2002) and Faure et al (J. Immune based Therapies and Vaccines 1: 2, 2003). WO 2009073631 A2 and Baer et al. (Infection and Immunity 77: 1083-1090) describes several engineered human antibody Fab fragments (amongst which Fab1A8) specific for P. aeruginosa PcrV protein and which compete with MAb 166 for binding to the same epitope on PcrV. These Fabs show potent neutralization activity against the P. aeruginosa Type III secretion system. KB001 (KaloBios, US) is a Humaneered™ anti-PcrV PEGylated antibody Fab′ fragment (Anti-PcrV Program Fact Sheet, KaloBios) that showed potent Type III Secretion System (TTSS) neutralizing activity in cellular cytotoxicity assays. KB001 is being developed for the prevention of Pa ventilator-associated pneumonia (VAP) and for the treatment of CF. Preliminary evidence of activity and safety has been demonstrated in both indications in Phase 12 trials conducted by KaloBios. It still remains to be determined, however, whether or not escape mutants will develop to this monospecific monoclonal antibody once administered to patients.
Taken together, the increased incidence in certain types of infections, the increased use of invasive devices in the hospital as well as the increased frequency of multi-resistant Pseudomonas strains, have clearly let to a shortage of treatment options for nosocomial Pseudomonas infections. Despite the above efforts, management of P. aeruginosa infection represents a difficult therapeutic challenge for critical care physicians (El Solh and Alhajhusain J. Antimicrobial Chemotherapy 64: 229-238, 2009). For patients with multi-drug resistant strains, very few clinical options remain. It is therefore considered imperative to discover and develop novel anti-Pseudomonas drugs to fill a dangerous void in the anti-bacterial armamentarium of the clinician (Malcolm and Heim Curr. Opin. Pharmacol. 9: 558-565, 2009).